Hoar frost

Depth hoar, imaged with light and with scanning electron microscopy.Hoar frost (also hoarfrost, radiation frost, or pruina) refers to white ice crystals, deposited on the ground or loosely attached to exposed objects such as wires or leaves.[4] They form on cold, clear nights when conditions are such that heat radiates out to the open sky faster than it can be replaced from nearby sources such as wind or warm objects. Under suitable circumstances, objects cool to below the frost point[5] of the surrounding air, well below the freezing point of water. Such freezing may be promoted by effects such as flood frost or frost pocket.[6] These occur when ground-level radiation losses cool air till it flows downhill and accumulates in pockets of very cold air in valleys and hollows. Hoar frost may freeze in such low-lying cold air even when the air temperature a few feet above ground is well above freezing.The name hoar comes from an Old English adjective that means "showing signs of old age"; in this context it refers to the frost that makes trees and bushes look like white hair.